Transfer of proteins and lipids by means of small, membrane-bound vesicles within the cell is essential for virtually all cell functions. Defects in targeting functional molecules to the appropriate destinations can render cells non-functional, thereby causing diseases. The Rab small G protein family (Rab) consists of over sixty members, and is implicated in intracellular vesicle transport, which includes exocytosis, endocytosis, and transcytosis. Rab cycles between the GDP-bound inactive form and the GTP-bound active form and translocates between the cytosol and the membranes, and these cyclical activation, inactivation, and translocation processes are regulated by at least three types of regulatory proteins (GDI, GEP, GAP). The GTP-bound form then interacts with downstream effectors and functions through them. Evidence is accumulating that Rab is a key molecule to clarify molecular physiology and pathophysiology of vesicle transport
